The present paper reviews current concepts on the role of cyclooxygenase (COX) in the development of malignant tumors. An inducible isoform of cyclooxygenase is expressed in neoplastic, pre-neoplastic, and peri-neoplastic cells by mutation of oncogenes (such as ras), tumor promoters, mitogens, cytokines, their receptors, and pathogenic factors such as Helicobacter. Cells overexpressing cox-2 escape apoptosis, have abnormal cell-to-cell interactions, and acquire invasive phenotypes. On the other hand, angiogenesis plays a key role in the development of malignant tumors. Both in vitro and in vivo studies indicate that cox-2 overexpression upregulates angiogenic factors in neoplastic cells and promotes tumor angiogenesis. It is also possible that cox-2 expression upregulates angiogenic factors in peri-neoplastic cells that express the isozyme. Interestingly, cox-1, the other isozyme that is expressed in tumor vascular endothelia, participates in tumor angiogenesis, because an anti-sense oligonucleotide of cox-1 suppresses in vitro angiogenesis induced by cox-2-overexpressing cells. A non-specific COX inhibitor, not a specific COX-2 inhibitor, reduced growth and angiogenesis in cancer xenografts by inhibition of COX-1 in vascular endothelial cells, even when the tumor did not express COX-2. These results demonstrate that COX inhibitors suppress angiogenesis and tumor growth by inhibiting expression of angiogenic factors and vascular endothelial cell migration. Furthermore, another concept is emerging to indicate that prostaglandins (COX-2 products and mediators of classic inflammation) suppress host immunity against tumors. This evidence supports the hypothesis that COX is an important perigenetic factor in the development of cancer growth, and offers a new strategy against cancer using COX inhibitors (nonsteroidal anti-inflammatory drugs).